Add both `[.]` and `[!hide]` tags when registering a hidden test case, as per documentation.

Co-authored-by: Martin Hořeňovský <martin.horenovsky@gmail.com>

 - Overrides added
 - usages of push_back() replaced with emplace_back()
 - Loop variable made const-refernce
 - NULL replaced with nullptr
 - Names used in the declaration and definition unified
 - size() replaced with empty
 - Identical cases merged

This way it is explicit when there is a `StringRef` -> `std::string`
conversion and makes it easier to look for allocations that could
be avoided.

Doing this has already removed one allocation per registered test
case, as there was a completely pointless `StringRef` -> `std::string`
conversion when parsing tags of a test case.

Self test baselines also modified accordingly, due to
one typo found in a string in test code.

std::isalnum expects an int in the range of unsigned char or -1 (EOF),
otherwise it exhibits undefined behavior. Casting from char to unsigned
char avoids this.

MSVC warns about this when compiling with /analyze.

* Eliminated some copies
* Made makeTestCase fit into 4 arguments -- avoids spills on Win64
* Made string literals into StringRef literals

ReusableStringStream holds a std::ostringstream internally, but only exposes the ostream interface.
It caches a pool of ostringstreams in a vector which is currently global, but will be made thread-local.

Altogether this should enable both runtime and compile-time benefits. although more work is needed to realise the compile time opportunities.